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    <title>Document</title>
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    <canvas id="canvasgl" width="800" height="600">
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    </canvas>
    
    <script src="../lib/webgl-utils.js"></script>
    <script src="../lib/webgl-debug.js"></script>
    <script src="../lib/cuon-utils.js"></script>

    <script>
      var VSHADER_SOURCE = `
              attribute vec4 a_Position;
              uniform mat4 u_xformMatrix;

              void main(){
                  gl_Position = u_xformMatrix * a_Position;
              }
          `;

      var FSHADER_SOURCE = `
              void main(){
                  gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
              }
          `;

      var canvas = document.getElementById("canvasgl");
      var gl = getWebGLContext(canvas);
      if (!gl) throw new Error("can't get webgl context");

      if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        throw new Error("can't init shaders");
      }

      var n = initVertextBuffers(gl);
      if (n < 0) throw new Error("failed set positions of vertices");

      let Tx = 0.2, Ty = 0.2, Tz = 0.0;
    //   let Tx = 0.0, Ty = 0.0, Tz = 0.0;
      var xformMatrix = new Float32Array([
          1.0, 0.0, 0.0, 0.0,
          0.0, 1.0, 0.0, 0.0,
          0.0, 0.0, 1.0, 0.0,
          Tx, Ty, Tz, 1.0
      ]);
      var u_xformMatrix = gl.getUniformLocation(gl.program, "u_xformMatrix");
      gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix);

      gl.clearColor(0.0, 0.0, 0.0, 1.0);
      gl.clear(gl.COLOR_BUFFER_BIT);
      gl.drawArrays(gl.TRIANGLES, 0, n);

      function initVertextBuffers(gl) {
        var vertices = new Float32Array([0.0, 0.5, 0.5, 0.5, 0, 0]);
        var n = 3;

        var vertexBuffer = gl.createBuffer();
        if (!vertexBuffer) throw new Error("can't init vertext buffet");
        gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
        gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

        var a_Position = gl.getAttribLocation(gl.program, "a_Position");
        gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
        gl.enableVertexAttribArray(a_Position);
        return n;
      }
    </script>
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